1. Field of the Invention
The invention relates to a stirrer, comprising a spindle provided with a drive, to which spindle a plurality of stirring blades are affixed, lying substantially in radially directed axial planes.
2. Description of the Prior Art
A stirrer of the above-mentioned type is disclosed in the French Patent Specification No. 1,315,140. Stirrers of this type are usually indicated as turbine stirrers. They are used for mixing gasses and/or liquids, the so-called aeration of liquids, or the homogenisation of fluids or keeping them in homogenised condition.
The mixing action of a normal turbine stirrer, consisting in principle of a number (usually 4, sometimes 6) of blades directed radially parallel to the axis of rotation, which are connected by spokes, jibs or similar means to the spindle, arises by two phenomena:
producing strong turbulence around the blades, and
producing a circulatory flow in the vessel as a whole.
This circulatory flow, also called pumping action, is the result of the centrifugal field of forces in the fluid between the rotating blades, by which said fluid is, as it were, swung outwardly against the static pressure.
The turbulence required for mixing arises by the whirls behind all blade-edges, these whirls alternately arising and growing there and finally disengaging from the blade and being washed along with the circulatory flow. So hereby just the opposite is attained from what is aimed at by streamlining; sharp, angular blades, having a stream against them more or less perpendicularly to their plane, have a high resistance and make the required great intensity of turbulency arise.
With most of the turbine stirrers, however, the centrifugal flow is weak, whereby the area in which a good mixing is produced remains limited to the direct neighbourhood of the blades. Furthermore one should beware for the creation of cavitation in the whirls behind the blade edges, and thereby the filling of the whirls with gas, when increasing the number of revolutions too high, by which the mixing action declines strongly. Also the energy consumption is relatively high because of the high resistance of the blades at small pumping action. At great viscosity of the medium to be stirred furthermore the pumping action is generally too small to get sufficient circulation of all liquid.
The above mentioned limited mixing behaviour of most turbine stirrers, having various stirring blade shapes, is the result of the irregular disengagement pattern of the whirls at the edges of the turbine blades. Thereby the centrifugal action is irregular and the turbulence produced is very chaotic. As a consequence the pumping action is small and the whirls produced break each other up rapidly so that they extinguish quickly.
The above mentioned embodiment, known from the French Patent Specification No. 1,315,140, having stirring blades which are trapezium shaped, so have more or less the shape of a delta wing, already has the advantage as compared with other turbine stirrers, that a regularly, conically growing tipping whirl is produced along the edge of each blade which goes down angularly toward the circumference, which whirl leaves the blade, as a corkscrew whirl, with the centrifugal flow at the outer circumference. By the favourable distribution of pressure in the whirl, cavitation on the blades is prevented. Furthermore, corkscrew whirls are very stabile so that, when there is a stronger circulation, they will penetrate far into the medium and assure a good mixing.
Such stronger circulation is a result of the favourable distribution of pressure in the whirls by which an improvement of the pumping action is obtained. By the stronger circulation and the favourable division of turbulent energy in the vessel, the energy consumption per unit volume of the product to be mixed will also become relatively low.